1. Field of the Invention
The present invention relates to a heat exchanger having a double pipe construction, which is applied to a vehicle air conditioning apparatus.
2. Description of Related Art
A general coaxial pipe for a heat exchanger includes, as shown in FIG. 8, an inner pipe 110, and an outer pipe 120 having plural projections 131. The projections 131 are formed at the inside wall of the outer pipe 120 integrally and press-connected to the outside wall of the inner pipe 110.
The coaxial pipe is manufactured by forming the inner pipe 110 and the outer pipe 120 independently, inserting the inner pipe 110 into the outer pipe 120, and ironing the outer pipe 120 to press-connect the projections 131 to the outside wall of the inner pipe 110.
However, in the conventional method, the forming process to form the inner pipe 110 and the outer pipe 120 independently and the ironing process are needed. Therefore, it is difficult to reduce the total cost for manufacturing the coaxial pipe.
Further, in the conventional manufacturing method in which the projections 131 are press-connected to the outside wall of the inner pipe 110 by ironing the outer pipe 120, when the coaxial pipe is bent for downsizing the heat exchanger, the projections 131 are likely to be separated away from the outside wall of the inner pipe 110 at the bent portions.
When the projections 131 are separated away from the outside wall of the inner pipe 110, heat transmitting ratio between first fluid flowing through the outer pipe 120 and second fluid flowing through the inner pipe 110 decreases, thereby reducing the heat-exchanging ability of the heat exchanger.